Trained Eye

 
 
 

Skylight Idea

Question:

We have a kitchen with only a north-facing window and it is rather dark.  One of the ideas that I had was to put in a skylight through the standard gable style roof.  The comments I get from some people are the skylight will leak and drip. Are skylights a poor idea in our climate? I was thinking of running a cold air duct up to the boxed-in area to get better air movement. This might solve the problem. Your thoughts would be appreciated.

Answer:

The advice you have been given is quite true, skylights leak. I like to tell my clients that it is not a case of if the skylights will leak, but when they will leak. This is mainly due to our climate, but also inherent difficulty in installation, in a tricky situation. This is not to say that skylights should never be installed, but you should be aware that at some point you will have to deal with the leakage and do repairs and regular maintenance to prevent this from happening or continuing, once occurring.

The first reason that many skylights leak is the nature of the skylight, itself. To understand this I will try to simply describe the installation procedure in an existing home. In a nutshell, a skylight is installed by cutting a hole through the roofing materials, underlay (if present) and roof sheathing. The rafters or trusses may also have to be cut and reinforced depending on the size of the skylight to be installed. Once this is done a curb is built that sticks up several centimetres above the roof and is sized accordingly to accommodate the new skylight. This is where the difficulty begins.

A raised curb is needed to prevent rainwater and snow runoff from seeping into the hole cut for the skylight, but this curb can also act like a dam that prevent this same water from easily running off the roof. It can also trap extra snow and may increase the chances of ice damming, especially if excess heat escapes the home through or around the new skylight. Because of this problem, the curb must be exceptionally well sealed to prevent leakage. This is normally done with a series of metal flashings, often combined with roofing membranes or underlays. The final step is to install new roofing or patch the existing shingles and seal everything well with roofing cement. The skylight itself is often caulked in place or self-sealing once installed on the top of the curb. It should also be screwed or fastened to prevent movement and allow removal, if needed.

Completion of this installation procedure can be quite complex or difficult depending on the location, pitch, and design of the area of the roof where the renovation is done. Additional insulation, air-vapour barriers and air sealing will also be needed to complete the installation. If all this is done well, which is certainly possible, the area may remain dry for many years. If the installation is done poorly in any one or more of these areas, chances are high that you will get a leak. Ensuring the experience and knowledge of the contractor installing the skylight is essential. It is very, very difficult for an inexperienced individual or novice to anticipate all the elements to consider for a properly sealed skylight installation on an existing home.

The second cause of “leaks” may cause substantial water problems but may not be a leak at all. You hit the nail on the head with your concern over air movement around the skylight box. The new box created in the ceiling is an ideal are for warm, moist air to become trapped, especially in a kitchen. This warm air will invariably condense on the underside of the cool skylight surface and may drip down, if not controlled. Many skylights have a small metal or plastic trough built into the design to catch this condensation as it runs down the inner surface of the dome or sealed unit. In most situations, this trough will catch sufficient water to allow proper evaporation, but not always. It is common for this trough to overflow and drip on to the light box or the floor below if excess moisture is present.

To prevent this, your idea of running a return air duct near the skylight may be very difficult to accomplish. Most return air ducts are installed as close to the floor as possible, to allow somewhat passive collection of the furnace-heated house air, once it has cooled and dropped to the area near the floor. A ceiling mounted register may not accomplish this and even cause other problems with air movement. A better solution is installation of a strong kitchen exhaust fan, vented to the exterior of the house. This will allow some of the excess moisture created from washing and cooking in the kitchen to be blown outside the home. Installation of a ceiling fan, near the new skylight, may also help with the air circulation in the area desired. Moving the air with a ceiling fan will help dry the moist air in the upper area of the kitchen and may substantially reduce condensation when used in conjunction with an exhaust fan.

 

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